1. Field of the Invention
The present invention is directed to x-ray generators, and in particular to x-ray generators having an electrically conductive liquid forming the anode (or anti-cathode).
2. Description of the Prior Art
So-called liquid anode x-ray tubes are described, for example, in U.S. Pat. No. 2,665,390 and in German Pat. No. 890,246. X-ray sources for medical diagnostics are disclosed in U.S. Pat. No. 4,357,555, in EP-A-0 136 762 and in Philips Tech. Rev. 41, 1983/84 No. 4, pages 126 through 134. The x-ray tubes known from J. Urlaub, Rontgenanalyse Vol. 1: Rontgenstrahlen und Detektoren (Siemens, Karlsruhe 1974) pages 71 through 75 are usually used for fine-structure examinations.
X-ray sources having high spectral brilliance are required for implementing highly sensitive x-ray analysis methods (total reflection x-ray fluorescence analysis, reflectometry, interferometry, diffractometry, etc.). Because synchrotrons, the most intense x-ray light sources known at present, are not available as laboratory sources, attempts have been made to enhance the brilliance of conventional x-ray tubes by applying the following techniques:
Diminishing the electron focus on the anode (increasing the power density of the electron beam)
Employing a rotating anode (distribution of the thermal stress onto the generated surface of a rapidly rotating anode)
Diminishing the effective x-ray emission area with a flat beam tap [sic] (see, for example, J. Urlaub, Rontgenanalyse Vol. 1, pages 96 through 98).
For fixed anodes as well as rotating anodes the brilliancy obtainable with these techniques is already exhausted to the limit values of the material. Moreover, the use of rotating anodes presents considerable technical difficulties because the rotary transmission lead-throughs required for the drive of the anode and for the circulation of the coolant must still reliably seal the coolant circulation and the evacuated tube housing even given speeds of up to 6000 rpm. Despite complicated designs, leaks repeatedly lead to outages. Moreover, the electron beam effects a high local heating of the anode, as a result of which the anode is subjected to extreme mechanical stress and therefore ages extremely rapidly. Cracks form with increasing operating duration. Due to the more pronounced self-absorption, this effects a loss of brilliance. Additionally, the cracks can lead to the leakage of coolant into the tube vacuum. The high local heating of the anode can also cause an evaporation of anode material and can lead to arcing given the high electrical field strengths.